Contact electrification is a scientific theory from the Enlightenment that attempted to account for all the sources of electric charge known at the time. It has been superseded by more modern notions. In the late 18th century, scientists developed sensitive instruments for detecting 'electrification', otherwise known as electrostatic charge imbalance. The phenomenon of electrification by contact, or contact tension, was quickly discovered. When two objects were touched together, sometimes the objects became spontaneously charged. One object developed a net negative charge, while the other developed an equal and opposite positive charge. Then it was discovered that 'piles' of dissimilar metal disks separated by acid-soaked cloth, Voltaic piles, could also produce charge differences. Although it was later found that these effects were caused by different physical processes - triboelectricity, the Volta effect, differing work functions of metals, and others - at the time they were all thought to be caused by a common 'contact electrification' process.